The present invention relates to a novel web and composite structure formed therefrom.
Composites are generally an assembly of dissimilar materials that work together to perform a function only possible in the composite form. Generally, composites include a resin matrix with a fiber reinforcement material. "Advanced composites" generally refer to newer materials forming the resin matrix or the fiber reinforcements in which the fibers possess a Young's Modulus of greater than 12 million.
Fibers can be constructed of Kevlar.RTM., carbon fiber, Nextel.RTM., boron, or any other materials having a very small diameter and high strength and stiffness. Resins may typically consist of an epoxy, polycyanate, bismaleimide, and the like. The strength and stiffness of the resin matrix also affects the strength of the finished composite structure. For example, stronger resins such as epoxies usually yield a higher strength composite structure than lower strength resins such as polyester.
Structural fibers are generally formed into yarns or rovings which include a number of twisted or untwisted strands either plied together or formed in a continuous filament. In the past, these yarns have been woven into a cloth entailing the application of a sizing or lubricant to achieve this condition. After weaving, the lubricant is removed and a surface finish is applied either to prevent or promote the adhesion of resins which are later applied during the pre-impregnation or assembly process. In this regard, reference is made to U.S. Pat. No. 4,534,919 which discloses a carbon fiber tow suited for resin impregnation having disrupted parallel filaments.
Unidirectional tapes have been constructed of carbon fibers or other fibers in a dry fiber form or in a form pre-impregnated with a resin matrix. For example Y.L.A Inc. of Benicia, Calif. produces a single ply unidirectional tape under the designation XN 50A/RS-3. Such unidirectional tapes have been used in layup processes for the fabrication of sporting goods and aeronautical structures such as wing skins, solar arrays in satellites and the like. Lack of transverse integrity limits formation of core structures from existing unidirectional tapes, ie; they are delicate and prone to splitting along the side-by-side fibers. Thus, such unidirectional tapes are difficult to handle and process into a honeycomb structure.
Structural fibers may be formed into finished composite structures either by employing woven or non-woven web reinforcing material. For example, U.S. Pat. No. 5,013,514 proposes production of a hollow element utilizing woven or non-woven carbon fiber mats. U.S. Pat. No. 3,255,062 shows a method of manufacturing a reinforced honeycomb structure utilizing foam, plastic, or cardboard. U.S. Pat. No. 3,200,489 teaches a method of making a honeycomb core using stainless steel which is expanded from multiple foils or sheets which are bonded together at points.
U.S. Pat. Nos. 3,248,275 and 3,137,604 describe a honeycomb structure formed of resins impregnated in glass cloth. U.S. Pat. No. 4,563,321 reveals a method of producing a unitary curved structure having a honeycomb core which employs woven fiber glass material and an outer layer of chopped glass fibers.
The use of honeycomb core materials for constructing lightweight panels or sandwich structures is well established in the aeronautical and spacecraft fields. For example, in commercial aircraft, nearly all of the movable control surfaces, wing and tail leading and trailing edge fixed surfaces, doors, and interior cabin structures employ panels formed of honeycomb cores. Such prior art cores have typically been constructed of an aluminum or Aramid paper (known as Nomex.RTM.) honeycomb. Although more expensive than simple structures, the honeycomb core panel possess equal strength at higher stiffness, lower weight, and is resistant to higher natural vibration frequencies. Such resistance is very important when structural elements are employed in close proximity to jet and rocket engines. Reference is made in this regard to treatises entitled "Composite Basics" second edition by A Marshall; International Encylopedia of Composities, Volume 1, pgs. 488-507, Lee; Handbook of Composites, chapter 21, G. Lubin; and a brochure entitled "Honeycomb, TSB 120" Hexcel Corp which describe honecomb cores in detail Moreover, the honeycomb core must have small enough cell sizes to provide stabilization of the facings against premature buckling. In addition, the core must be sufficiently tough and abuse resistant to enable the same to be easily handled in a fabrication shop.
Aramid honeycombs are used where high damage tolerance and abuse resistance is a criteria. However, Aramid honeycombs lack the shear and compressive strength of aluminum honeycombs.
Aluminum, the presently preferred core material for minimum weight primary structures in spacecraft and aircraft, also possesses problems in that using the same at one pound per cubic foot density provides ample strength for the primary loading of a structure, but results in a very fragile structure which is easily damaged when subjected to the normal manufacturing, assembling and testing procedures used in fabrication. In addition, aluminum cores do not provide a compatible coefficient of thermal expansion relative to the facing material which is normally a carbon fiber. As a result, changes in temperature result in the warpage of the structure. Such warpage can occur during the panel manufacturing process as a result of cool-down from the core-facing bonding temperature to room temperature, typically a 275 degree fahrenheit difference. Also, warpage occurs in outerspace if such a panel is employed as a spacecraft structure when the spacecraft moves from daylight to darkness and back again.
A lightweight, thin, web having unidirectional structural fibers for constructing lightweight honeycomb cores would be a great advance in the field of materials technology.